yep its call "tubercles" and only really starts to show on males [all types] around 2 of age which is prime for breeding while they are in breeding mode which is just above 20 degree really.
since its ur first time breeding goldfish i would suggest just let them do it normally. select a nice size female and 2 to 3 male for breeding keep the males and the female separate for conditioning.
keep the males on a normal diet and put the female on a high protein diet like meal worm, brine shrimp....... and just as the female is about to burst put some spawning mop in with the female and introduce the males.
now do a cold water change and see the males chase after the female and do prepare yourself as they're pretty ruff and u may need to remove the female to make them stop. as soon as u think u have enough eggs take all the parents out and drop the water level down to around 12cm and wait 4 to 5 days before hatching.
be prepare to do some kulling as a medium size female can easily drop over 1000 eggs and a lot of them will be nymphs.
i am no expert but i do breed all sort of gold fish when they are in season.